An organic electroluminescent device, which is considered as a promising next-generation flat panel display to replace a liquid crystal display (LCD) and a plasma display panel (PDP), generally includes multi-layered organic compounds of illuminants, and emits light as a current flows when a voltage is applied thereto. Such an organic electroluminescent device is also called an organic electroluminescent display (OELD) or an organic light emitting diode (OLED).
While an LCD displays an image by selectively transmitting light and a PDP displays an image through plasma discharge, an OLED displays an image through electroluminescent mechanism. That is, an OLED includes two electrodes, i.e., cathode and anode, and an organic luminescent material interposed therebetween, and emits light in such a way that holes and electrons are injected into an organic luminescent layer from the anode and cathode, respectively, and then recombined with each other to create a recombination energy stimulating organic molecules. Such an OLED is being popularly applied to small-sized displays because it is self-luminescent and also has several advantageous merits such as wide viewing angle, high-definition, high-quality image and high response time. In particular, the OLED is being focused as next-generation displays for televisions and flexible displays.
Recently, an active matrix organic light emitting diode (AMOLED) is being predominantly used among various OLEDs because it adopts an active driving method capable of individually controlling pixels that are minimum units forming a screen. In the AMOLED, a scanning line is formed in one direction, and a signal line and a power supply line are formed in another direction crossing the one direction, and thereby one pixel area is defined. A switching thin film transistor (TFT) connected to the scanning lines and the signal lines, a storage capacitor and a driving thin film transistor are formed in the pixel area. The storage capacitor is connected to the switching thin film transistor and the power supply line, and the driving thin film transistor configured to supply current is connected to the storage capacitor and the power supply line. And the OLED is formed to be connected to the driving thin film transistor.
In the AMOLED that supplies current through a TFT, carrier mobility becomes poor if an active layer used as a channel of the TFT is formed of amorphous silicon. Accordingly, polysilicon is used for the active layer because it has a high carrier mobility of 10 cm2/Vsec or more. However, a fabrication process of a polysilicon TFT is performed at a very high temperature, which makes it difficult to fabricate a flexible display.
Since a glass substrate used to fabricate the TFT is deformed at a high temperature of 600° C. or more, a low temperature polysilicon (LTPS), which is obtained by crystallizing amorphous silicon at a low temperature, has been proposed to form polysilicon. To crystallize the amorphous silicon, an excimer laser annealing (ELA) process has been suggested. According to the ELA process, however, an overlap region onto which laser is duplicately irradiated may appear so that the overlap region differs in crystallization degree from other regions where laser is not duplicately irradiated. This leads to a decrease in stability. For example, current amount of a TFT may be varied depending on device characteristics changed by crystallization degree. In particular, the current amount of the TFT may be varied depending on threshold voltage difference. Further, each TFT has a different grain boundary, and thus has nonuniform electrical properties.
Amorphous silicon or polysilicon should be formed in a typical display because the typical display employs TFTs. Further, a TFT configured to compensate for a current should be formed because an OLED is driven by a current. Because four or six TFTs should be used at present, a fabrication process becomes too complicated and fabrication cost is increased. Moreover, the TFT decreases an aperture ratio in a bottom emission type device so that the aperture ratio is considerably reduced to 30-50%.
A related art display has only one current level, and thus a voltage should be divided into 64 levels to realize 64 gray scale levels using only one current level. However, the related art display has a limitation in that it is difficult to realize a desired gray scale if a dividable voltage level is too narrow.